Trans-acting factors that mediate B cell specific transcription of immunoglobulin (Ig) genes have been postulated based on an analysis of the expression of exogenously introduced Ig gene recombinants in lymphoid and non-lymphoid cells. Two B cell-specific, cis-acting transcriptional regulatory elements have been identified. One element is located in the intron between the variable and constant regions of both heavy and kappa light chain genes and acts as a transcriptional enhancer. The second element is found upstream of both heavy chain and kappa light chain gene promoters. This element directs lymphoid-specific transcription even in the presence of viral enhancers.
Mouse and human light chain promoters contain the octamer sequence ATTTGCAT approximately 70 base pairs upstream from the site of initiation. Heavy chain gene promoters contain the identical sequence in inverted orientation, ATGCAAAT, at the same position. This element appears to be required for the efficient utilization of Ig promoters in B cells. The high degree of sequence and positional conservation of this element as well as its apparent functional requirement suggests its interaction with a sequence-specific transcription factor but no such factor has been identified.